Toward Molecular Imaging with Xenon MRI

When it comes to molecular imaging—the burgeoning discipline of visualizing biological processes at the cellular and molecular level in living organisms—magnetic resonance imaging (MRI) is low on the list of favored techniques (1). The receptors and gene expression products that we aim to observe by molecular imaging are present at minuscule concentrations (around 10−6 to 10−12 mole per liter) in the body. Yet the nuclear magnetic moments that provide the signal in MRI are tiny, and lots of them are needed to generate an image. MRI is thus limited to detecting concentrations of 10−3 to 10−5 mole per liter. In contrast, radioisotope imaging methods such as positron emission tomography and single photon emission tomography can detect probe molecule concentrations of 10−9 to 10−12 mole per liter. These techniques have been the natural choice for molecular imaging, although they lack the spatial and temporal resolution of MRI and make use of ionizing radiation.